1. Field of the Invention
This invention relates to the control of the volume of intake air for an engine by controlling the opening of a throttle valve provided in an inlet pipe for a car engine. More specifically, it relates to a method and device for controlling the volume of intake air for an engine having two accelerator opening detection means for detecting the operation amount of an accelerator.
2. Description of the Prior Art
In a general car engine control device, a throttle valve is provided in an inlet passage for sucking air to be supplied to an engine and opens or closes in response to the operation of an accelerator by a driver to control the volume of intake air for the engine in accordance with the operation amount of the accelerator. The control of the volume of intake air for the engine can be achieved by connecting the throttle valve and the accelerator by mechanical connection means such as a link or wire. However, in a connection method using such mechanical connection means, the relationship between the operation amount of the accelerator and the opening of the throttle valve is fixed, there is no freedom in the control of the throttle valve, and the positional relationship between the throttle valve and the accelerator is restricted, whereby the installation position of the connection means is limited when installed on an automobile.
A constant-speed running control device, traction control device and the like have recently been installed on an automobile to automatically control the running of the automobile. Therefore, the throttle valve must be controlled independently of the operation of the accelerator by the driver, and the control of the throttle valve is carried out by electrically connecting the throttle valve to a motor or the like. When the throttle valve and the accelerator are not interconnected by mechanical connection means, an accelerator position detection sensor is provided near the accelerator to detect the operation amount of the accelerator by the driver, and the throttle valve is controlled based on the detected operation amount of the accelerator.
Since an electronic control device for an automobile has a larger number of parts than the case where mechanical connection means is used, failure probability increases. Therefore, to ensure safety driving, various safety measures such as the detection of a failure are taken. Particularly, in a device for controlling the volume of intake air for an engine which drives and controls the throttle valve with an electric signal, the detection of a failure of an accelerator position detection sensor is a serious problem and a failure of the accelerator position detection sensor causes an abnormal rise in engine speed or abnormal acceleration of a vehicle.
To cope with these problems, Examined Japanese Patent Publication No. Hei 6-94820 discloses a method for detecting a failure of an accelerator position detection sensor which checks if an accelerator position signal is appropriate with two position sensors for outputting a signal indicative of the position of the accelerator. In the first invention of the publication, a first position sensor composed of a potentiometer and a second position sensor composed of an accelerator switch are provided, the second detection sensor generates a switching signal when it detects the operation of the accelerator, and an error signal is generated when the output value of the first position sensor is not between two different threshold values larger than an idle value when this switching signal is generated. In the second invention, the first position sensor and the second position sensor are composed of a potentiometer, and an error signal is generated when a difference between the output value of the first position sensor and the output value of the second position sensor exceeds a predetermined maximum value.
However, since it is difficult to specify a malfunctioning sensor by the detection of a failure of the accelerator position detection sensor, it is impossible to judge which output signal value from the accelerator position detection sensors is used to set the target opening of the throttle valve at the time of emergency driving after the judgment of a failure to ensure safety driving. Therefore, safety cannot be ensured. To specify a malfunctioning sensor, it is conceivable to install at least one additional sensor. This may cause such problems as growth in failure rate due to an increase in the number of parts, complicated failure judgment logic and a cost increase due to the addition of a sensor.